It has been proposed to produce magnesium in an electrolytic process by using as the anode a mixture of a reducing agent such as carbon, coke or graphite with the carbonates, hydroxides or oxides or magnesium. Exemplary of such proposals are British Pat. Nos. 452,269, 473,068, 5211,076 and 825,872. Cells employing such composite anodes offer the advantage of eliminating the need for a separate chlorination step to produce the magnesium chloride feed stock; they also eliminate the generation of chlorine gas in substantial quantities during the electrolytic process. These results are believed to be achieved because the nascent chlorine generated during the electrolytic decomposition immediately enters into reaction with the mixture and thereby produces in situ the chloride of the metal to be produced.
Notwithstanding the potential advantage of such arrangements, magnesium cells using composite anodes as described in the aforementioned patents have not met with any degree of commercial success. In large part, the lack of success has been due to the highly resistive nature of the composite mixture and the attendant large cell voltages, e.g., 20 volts, required to achieve current densities adequate for the commercial production of magnesium. Thus, the power consumption of such cells has been a major contributing factor in the lack of any commercial application of such cells.
Some efforts have been made to overcome the high resistance of these composite anodes. For example, British Pat. No. 511,076 suggests that conductor rods be laterally inserted in the anode and positioned near the surface of the bath to reduce the voltage losses. That arrangement, however, still leaves relatively long current paths through the highly resistive mixture to the active anode surface and has not, therefore, been a satisfactory solution to the problem.
Another proposal is described in British Pat. No. 825,872 in which a composite mixture to be used in a self-baking anode is supported by an outer metal shell and an internal reinforcing metal lattice work, both of which are connected to a source of electric power. However, since current flows through the path of least resistance, the principal current flow in that arrangement would be from the outer shell laterally through the electrolyte to the adjacent cathode. The inefficiency of the arrangement disclosed in this patent is apparent from the requirement that cell voltages be in the range of 10 to 20 volts. Cell voltages at that level are far in excess of voltage levels presently achieveable with existing commercial magnesium cells.